2. • Located between the San Juan and Sangre de Cristo
Mountains in Southwestern Colorado’s San Luis
Valley
• Over 12,000 to 130,000
years old (exact time they formed is unknown)
• Approximately 42,000 acres
• Composed of black magnetite, pink feldspar, red and
tan sandstone, green epidote, white quartz, and
many other minerals.
3. Medano Creek
Fluvial Process
• Seasonal stream that runs through the national
monument
• Created from mountain run off and seasonal rain,
this determines the volume of flow.
• Moisture from the creek keeps dunes stabilized so
they don’t move
• “Conveyor belt for sand”- flow carries sand
downstream and when the creek dries up the wind
blows the sand back to the dunes
5. Wind (aeolian) Transportation
Arid Land
• Dunes are created by a combination of wind patterns.
Northeast wind blows strongly across the valley, weaker
winds blow southwest through a gap in the mountains
• Particles are moved by saltation and traction or in other
words bounced and rolled
• The surface layer moves slowly downward as a result of
saltation, this is called creep
• When the sand is deposited (known as aeolian
deposition) it creates a sand dune
8. Where all the Sand Came From
Glacial Modification of Terrain
• During the Pleistocene Epoch the San Luis Mountains were covered
in glaciers
• Relatively warm summers during this time and the melting glacier
seasonally flooded the valley
• Glacial plucking occurred as the glaciers moved down the mountain
which is when the glacier picks up rocks and sand
• Meltwater carried tons of sand that the glaciers had scoured from
the mountains and deposited it in an alluvial fan
• Winter winds carried the sand across the valley
• The Sangre de Cristo Mountains blocked the wind and the sand
piled up forming the dunes.
9. Glacier of a Mountain Glacial Plucking Alluvial Fan
Caused from Meltwater
Sand Dunes are Formed Wind Blowing and
Moving Sand
